Recently, thin display devices, including flat-panel displays such as liquid crystal displays or organic light-emitting diode displays, are receiving a lot of attention. In particular, these thin display devices are provided in the form of a touch-screen panel, and are widely useful for a variety of smart devices characterized by portability, including not only smart phones and tablet PCs but also various wearable devices.
Such display devices in portable touch-screen-panel form are configured such that a window film for display protection is placed on a display panel in order to protect the display panel from scratches or external impacts. In most cases, reinforced glass for displays is used as the window film. The reinforced glass for displays is thinner than typical glass, but is manufactured so as to have high strength and high scratch resistance.
However, reinforced glass is heavy and is thus unsuitable for use in lightweight portable devices and is also difficult to realize break-resistance due to its poor ability to withstand external impact. Moreover, such reinforced glass cannot be curved beyond a predetermined level, making it unsuitable to serve as a flexible display material having bending or folding capability.
Thorough research is ongoing into optical plastic covers having strength or scratch resistance corresponding to those of reinforced glass while ensuring flexibility and impact resistance. The optical transparent plastic cover material, which is flexible compared to typical reinforced glass, may include polyethylene terephthalate (PET), polyether sulfone (PES), polyethylene naphthalate (PEN), polyacrylate (PAR), polycarbonate (PC), and polyimide (PI). However, these polymer plastic substrates may exhibit poor hardness and scratch resistance and insufficient impact resistance compared to reinforced glass for use in the window film for display protection. Hence, many attempts have been made to attain desired properties by coating these plastic substrates with a composite resin composition.
Thus, high hardness is ensured by forming a hard coating on a plastic substrate film. A typical hard coating is formed using a composition comprising a resin having a photocurable functional group, a curing agent or a curing catalyst and other additives. In particular, a composite resin having a polyfunctional group is applied on an optical plastic substrate film, whereby the resulting substrate film is suitable for use as a window for display protection having improved hardness and scratch resistance.
However, a typical photocurable composite resin is disadvantageous in that it is difficult to achieve high hardness comparable with that of reinforced glass and in that curling may significantly occur due to shrinkage upon curing. Furthermore, such a composite resin does not exhibit sufficient flexibility, and is thus unsuitable for use in a protective window film for a flexible display device.
Korean Patent Application Publication No. 10-2013-0074167 discloses a plastic substrate having high hardness and flexibility. In this case, however, a hard coating layer formed on the substrate merely functions to increase hardness and chemical resistance, and does not increase flexibility.